Bovine babesiosis is a tick-transmitted, hemoparasitic diease caused by intraerythrocytic protozoa belonging to the genus Babesia. The disease caused by Babesia manifests itself clinically by fever and extensive hemolytic anemia that often leads to hypotensive shock, cerebral involvement, and death. More than a half billion cattle are estimated to be at risk of acquiring babesiosis. This disease represents a primary impediment to food and fiber production in much of the world.
To date, control of bovine babesiosis in enzootic areas has been partially successful through vaccination with attenuated strains of Babesia spp. or with more virulent strains followed by chemotherapeutic control. Protective immunity in babesiosis may be directed against one or more surface antigens associated with sporozoites, infected erythrocytes, and/or merozoites. Merozoite surface antigens are important in the pathogenesis and immunology of babesiosis due to their role in the parasite's recognition of, attachment to, and penetration of host erythrocytes and their accessibility to the immune system.
Recently, progress has been made toward the identification and characterization of specific immunogens of merozoites of Babesia bovis (Smith, R. D., M. A. James, M. Ristic, M. Aikawa, and C. A. Vega Y Murgula [1981] Science 212:335-338; Wright, I. G., B. V. Goodger, K. Rode-Bramanis, J. S. Matlick, D. F. Mahoney, and D. J. Waltisbuhl [1983] Z. Parasitenkd. 69:703-714; Wright, I. G., G. B. Mirre, K. Rode-Bramanis, M. Chamberlain, B. V. Goodger, and D. J. Mahoney [1985] Infect. Immun. 48:109-113; Commins, M. A., B. V. Goodger, and I. G. Wright [1985] Int. J. Parasitol. 15:491-495; Wright, I. G. and P. W. Riddles [1986] "Biotechnological Control of Tick-Borne Disease," Meeting of the Food and Agriculture Organization of the United Nations, 6-10 Oct. 1986, pp. 1-21, Rome, Italy; Waltisbuhl, D. J., B. V. Goodger, I. G. Wright, G. B. Mirre, and M. A. Commins [1987] Parasitol Res. 73:319-323; Goff, W. L., W. C. Davis, G. H. Palmer, and T. C. McGuire [1988] Infect. Immun. 56:2363-2368) and Babesia bigemina (McElwain, T. F., L. F. Perryman, W. C. Davis, and T. C. McGuire [1987] J. Immunol. 138(7):2298-2304). However, in only one instance (Smith et al., 1981) were antigens which provided protection against infection determined to be surface-exposed on merozoites as opposed to cytoplasmic in location.
Bovine babesiosis can be caused by either Babesia bigemina or Babesia bovis. These parasites have antigenic similarities and differences that may have important functional roles in the induction of protective immunity and antibody-based diagnosis. Also, B. bovis isolates, including the current Australian vaccine strain, are now known to consist of subpopulations that vary antigenically, in virulence, and in abundance within an isolate (Cowman, A. F., P. Timms, and D. J. Kemp [1984] Mol. Biochem. Parasitol. 11:91-103; Gill, A. C., A. F. Cowman, N. P. Stewart, D. J. Kemp, and P. Timms [1987] Exp. Parasitol. 63:180-188).
Current vaccine strategies include the use of attenuated live Babesia bovis parasites and various inactivated preparations (Montenegro-James, S., M. Toro Benitez, E. Leon, R. Lopez, and M. Ristic [1987] Parasitol. Res. 74:142-150; Smith et al., 1981; U.S. Pat. No. 4,762,711 issued to Buening et al.; Kuttler, K. L., M. G. Levy, M. A. James, M. Ristic [1982] Am. J. Vet. Res. 43(2):281-284). The attenuated vaccine provides the best protection against challenge with both homologous and heterologous strains, although there are a number of serious disadvantages, including a short shelf-life, variation in virulence, contamination with host erythrocyte stroma, and perpetuation of the life cycle by creation of a carrier state. Inactivated vaccines induce protection against challenge with homologous strains; however, only partial protection occurs against challenge with heterologous strains.
Animals that survive natural field infection or that recover from infection with an attenuated vaccine strain are protected against clinical disease. However, premunization in this manner is expensive, impractical in developing countries that lack the necessary infrastructure, and a potential mode of transmission for other blood-borne diseases.